1. Field of the Invention
This invention pertains generally to processing of fresh fruit and vegetables, and more particularly to a method of preventing browning induced by wounds to plant tissue.
2. Description of the Background Art
Browning of fresh fruits and vegetables reduces quality and is often the factor limiting shelf life and marketability. This is especially true when these horticultural commodities are wounded by cutting, peeling, or abrading the surface during the preparation of minimally processed fresh fruits and vegetables. Enzymatic and non-enzymatic reactions with phenolic compounds produce brown pigments in plant tissue. Some tissues (e.g., artichokes) contain high levels of preformed phenolic compounds and rapidly brown in the air after wounding. Preventing browning in these tissues requires deactivation of the enzymes responsible for browning (e.g., polyphenoloxidase), exclusion of oxygen (e.g., oxygen levels below 1%), or application of chemical antioxidants (e.g., ascorbic acid). In other tissue, (e.g., lettuce) the quantity of phenolic compounds in uninjured tissue is low and browning follows the enhanced synthesis and accumulation of phenolic compounds.
Wounding (e.g., cutting, cracking or breaking) of lettuce produces a signal that migrates through the tissue and induces the synthesis of enzymes in the metabolic pathway responsible for increased production of phenolic compounds. The first enzyme in the phenylpropanoid pathway is phenylalanine ammonia-lyase (PAL). Induced synthesis of this enzyme after wounding is rapidly followed by the accumulation of phenolic compounds like chlorogenic, isochlorogenic and dicaffeoyl tartaric acid, compounds that are associated with browning in lettuce.
Methods used to control the increase in phenolic metabolism that leads to browning and loss of quality of minimally processed fresh produce include the use of reducing agents, enzyme inhibitors, acidulants, and complexing agents. Some of these chemical treatments are very effective in controlling browning by interfering with specific metabolic pathways. For instance, o-benzylhydroxylamine, cysteine, and some phenylalanine analogues (e.g., 2-aminoindan-2 -phosphonic acid) have been reported to reduce the activity of enzymes associated with phenylpropanoid metabolism (e.g., PAL). However, concern about the use of chemicals and their toxic nature precludes their use on many minimally processed fresh fruits and vegetables. This concern with chemical residues is eliminated by the use of low oxygen, and or high carbon dioxide controlled and modified atmospheres (CA and MA), which are treatments currently employed in the commercial packaging of minimally processed lettuce. However, the use of CA requires special equipment for handling and storage, while the use of MA requires special equipment for packaging and expensive packaging material.
Therefore, there is a need for a method that counteracts these effects and prevents browning without the use of chemicals or additives, and which is easy and inexpensive to implement. The present invention satisfies those needs, as well as others, and generally overcomes the problems associated with preventing of browning using chemicals or additives.